Tropomyosin from rabbit skeletal muscle has been separated into fractions containing only 34 000 dalton chains (SB-TM) and fractions containing both 34 000 and 36 000 dalton chains (DB-TM). Both SB-TM and DB-TM formed crystals and tactoids with a common 400 Å repeat. In addition the SB-TM gave a new crystal type which has not been described in studies on unfractionated tropomyosin. TN-T interacted with both tropomyosin forms to give a variety of geometric patterns, some of which have been previously ascribed to tropomyosin alone. TN-I inhibited tropomyosin tactoid formation, suggesting some type of direct interaction. TN-C, however, had no effect on the formation of tropomyosin crystals or tactoids or on their appearance.